1. Field of the Invention
The present invention relates to a solid-state image sensing apparatus for obtaining an electrical signal which is in accordance with a quantity of incident light and, more particularly, to a solid-state image sensing apparatus capable of controlling a direct current component of an output signal.
2. Description of Related Art
U.S. Pat. No. 5,241,575 has proposed a solid-state image sensing apparatus which logarithmically converts and outputs a photoelectric current generated according to the quantity of incident light. In the solid-state image sensing apparatus, a voltage is applied as a reference voltage for charge integration. However, since the reference voltage is constant, the direct current component of the output voltage from the solid-state image sensing apparatus varies depending on the quantity of incident light.
U.S. Pat. No. 4,763,200 has proposed an apparatus which determines, in order to obtain a stable image, a reference voltage for an A/D converter based on an output resulting from the previously-performed detection of the light quantity of an exposure source and on an output of an image sensing device when an irradiation of a standard pattern by the exposure source is sensed by the image sensing device.
In a solid-state image sensing apparatus, it is necessary to convert the output signal from analog to digital format. In A/D conversion, it is preferable to convert an output signal of a wider range in larger gradations. However, in the solid-state image sensing apparatus of the prior art, since the direct current component of the output signal varies depending on the light quantity, and the luminance signal of the subject is added with the direct current component as the reference, considering the output when the subject is bright and the output when the subject is dark, the range of the output signal is large as shown in FIG. 1, even if the luminance difference in the subject is small. FIG. 1(a) indicates the output when the entire subject is bright and, in this case, the direct current component is large. FIG. 1(b) indicates the output when the entire subject is dark and, in this case, the direct current component is small. The range indicated by Y is the luminance difference in the subject, and the range indicated by AD is a range to be converted by the A/D converter. In order to obtain large gradations from an output having such a large range, an A/D converter of a larger bit number is required. The more bit numbers an A/D converter has, the more expensive it is. The direct current component corresponds to the average value of waveforms within the range indicated by Y.
In the apparatus of the prior art, in order to determine the reference voltage for the A/D converter, a number of elements, and complicated processing are required. That is, a shading circuit, a line random access memory (RAM), a central processing unit (CPU), etc. are needed in addition to the image sensing device and the light quantity sensor. To add the output of the light quantity sensor and the output of the image sensing device, the phases of the outputs are synchronized before they are inputted to an adding circuit. Moreover, it is necessary to adjust the line RAM until its content takes a predetermined value.